Horse Power vs Compression Ratio

[Pamela H.]

Everything else being equal in an engine rebuild...
How much extra HP do we achieve by increasing the compression ratio by 2 full points?

[Joe C.]

Re: Horse Power vs Compression Ratio

The rule of thumb is a difference of about 3% per full point of static compression ratio. This assumes everything else being equal, especially spark advance program.

[Duke W.]

Re: Horse Power vs Compression Ratio

It depends on what you start with, but going from 8:1 to 10:1 will improve torque and power, ACROSS THE ENTIRE REV RANGE by 6-10 percent. It will also reduce fuel consumption by about the same amount in normal driving.

A more detailed discussion of the issue is here:

http://www.metroli.org/pdf/2012%20Nationals%20tuningseminar.pdf

Duke

[Timothy B.]

Re: Horse Power vs Compression Ratio

Also to consider is when you increase compression, you increase heat so the timing curve should be addressed. Depending on the camshaft selection, my thinking is that more heat requires less advance so it's best to experiment with the timing curve.

[Duke W.]

Re: Horse Power vs Compression Ratio

The "heat" introduced into an engine is the fuel's chemical energy, and this heat is released during combustion, so it's not a function of compression ratio.

What IS a function of compression ratio is the amount of this heat energy that is converted to useful mechanical work as expressed by thermal efficiency, and the higher the CR, the higher the efficiency. The loss of thermal efficiency in a lower compression ratio engine shows up as higher EGT due to the lower expansion ratio.

Lower compression may require a bit more spark advance due to lower mixture density, but from a practical standpoint in the range of compression ratios we a dealing with the difference is noise level.

We know from decades of experience what the optimum spark advance range is for all vintage Corvette engines for all operating conditions, and given available fuel octane, spark advance on the threshold of detonation is usually in the proper range for most configurations and will produce the highest torque/power across the rev range and the lowest fuel consumption.

You should review the presentation referenced in post #3 to gain a better understanding of the thermodynamic foundation behind internal combustion engine. It also includes recommended starting point spark advance maps for various OE engine families, but due to the variations in operating conditions that effect detonation such as air inlet temperature and altitude, it's best for each owner to do some experimenting and road testing to determine the most aggressive detonation-free spark advance map that can be achieved for their typical driving conditions.

Duke

[Mike E.]

Re: Horse Power vs Compression Ratio

Duke,
Your Power Point really opened my eyes about all the misconceptions and bad information about higher compression engines. Thanks You!

Back in 1971 when compression was lowered to accommodate no-lead& low-lead fuels what would the PON rating been for those fuels?

I’ve always been curious about when the Gen II (LT-1) was released there was much talk about reverse flow cooling that allowed higher compression. Was it a combination of reverse flow cooling and better spark timing management that allowed higher compression in that engine?

Mike

[Michael W.]

Re: Horse Power vs Compression Ratio

Back in 1971 when compression was lowered to accommodate no-lead& low-lead fuels what would the PON rating been for those fuels?

Ha! Trick question. PON didn't exist in 1971- just RON and MON.

[Duke W.]

Re: Horse Power vs Compression Ratio

Back when GM decreed that all engines beginning in 1971 would have to run on unleaded gasoline, the oil industry said the best that could do was 91 RON. So what would be the modern PON/AKI of that 91 RON fuel? The answer is on the slide titled "Fuels".

The LT1's reverse cooling seemed like a good idea. Since the cylinder head, especially near the exhaust valve, is the hottest area of the combustion chamber boundary, it is the most likely source of detonation, so cooling it first seems like a good idea, but it also removes heat from the engine, which might otherwise be converted to useful mechanical work.

From a practical standpoint, modern cooling systems circulate coolant very rapidly, so the difference in outlet and inlet temperature may be as little as ten degrees F. In this case does cooling system flow direction make any difference?

I never heard any reason why GM moved away from reverse flow cooling with the LS1. It just quietly went away.

The higher allowable compression ratio of the LT1 was probably more a function of the detonation sensor and spark advance control system than cooling. As I stated in the presentation, most modern engines are set up with high compression relative to available fuel octane and under many operating conditions are on the edge of detonation. This yields best output and fuel economy.

Since we have simple mechanical systems to control spark advance on our vintage engines that can't change the spark advance 50 times per second as on a modern engine, we have to leave a little margin to avoid significant detonation for worst case operating conditions, like high inlet air temperature, high coolant temperature, and high air density, which we can encounter in low speed driving during the summer season.

If you look at a modern Corvette owner's manual you will find that is says something like "premium fuel recommended, but not required" for the base engine. So you can run 87 PON and the detonation sensor will simply default to a less aggressive spark advance map and continue to monitor detonation. The same also applies to most modern cars, and most owners can't detect a difference in either performance or fuel economy.

There are exceptions like LS7 ("91 PON minimum") and most boosted engines. They probably will operate without damage on 87, but the loss of performance and/or fuel economy would probably be noticeable.

Duke

[Clem Z.]

Re: Horse Power vs Compression Ratio

Back when GM decreed that all engines beginning in 1971 would have to run on unleaded gasoline, the oil industry said the best that could do was 91 RON. So what would be the modern PON/AKI of that 91 RON fuel? The answer is on the slide titled "Fuels".

The LT1's reverse cooling seemed like a good idea. Since the cylinder head, especially near the exhaust valve, is the hottest area of the combustion chamber boundary, it is the most likely source of detonation, so cooling it first seems like a good idea, but it also removes heat from the engine, which might otherwise be converted to useful mechanical work.

From a practical standpoint, modern cooling systems circulate coolant very rapidly, so the difference in outlet and inlet temperature may be as little as ten degrees F. In this case does cooling system flow direction make any difference?

I never heard any reason why GM moved away from reverse flow cooling with the LS1. It just quietly went away.

The higher allowable compression ratio of the LT1 was probably more a function of the detonation sensor and spark advance control system than cooling. As I stated in the presentation, most modern engines are set up with high compression relative to fuel octane and under many operating conditions are on the verge of detonation. This yields best output and fuel economy.

Since we have simple mechanical systems to control spark advance on our vintage engines that can't change the spark advance 50 times per second as on a modern engine, we have to leave a little margin to avoid significant detonation for worst case operating conditions, like high inlet air temperature, high coolant temperature, and high air density, which we can encounter in low speed driving during the summer season.

Duke

my guess they did away with cooling the heads first as the longer warm up caused emission problems. now days they cool the blocks first and the heads later as you get heat from the heater in just a few blocks

[Joe C.]

Re: Horse Power vs Compression Ratio

There would be a HUGE difference in thermal efficiency between an engine equipped with a 929 cam and built with "10:1 compression" and an engine equipped with a 346 cam built with "10:1 compression".

When talking about thermal efficiency as a function of "compression ratio", be careful. You should not be talking about static compression ratio (SCR), you should be talking about the actual compression ratio. The actual compression ratio at low engine speeds is more accurately defined as the dynamic compression ratio, or DCR. The SCR is the volume of the engine's cylinder (bore x stroke) plus the clearance volume (combustion chamber, plus/minus piston to deck dimension, plus/minus piston crown volume/plus head gasket compressed thickness, plus ring land volume) divided by the clearance volume (combustion chamber, plus/minus piston to deck dimension, plus/minus piston crown volume/plus head gasket compressed thickness, plus ring land volume). The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the total cylinder volume (engine displacement/# of cylinders). This does not take into account the position of the piston at the intake valve closing event, which is needed to calculate the true ratio of the uncompressed volume to that of the compressed volume.

The actual compression ratio, or DCR, is calculated as follows (there are numerous online calculators that will do this for you):

• RD = Rod horizontal Displacement in inches
• ICA = advertised Intake Closing timing (Angle) in degrees ABDC
• RR = Rod Distance in inches below crank CL
• RL = Rod Length
• PR1 = Piston Rise from RR in inches on crank CL.
• PR2 = Piston Rise from crank CL
• ST = STroke
• 1/2ST = one half the STroke
• DST = Dynamic STroke length to use for DCR calcs

First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).

• RD = 1/2ST * (sine ICA)
• RR = 1/2ST * (cosine ICA)
• PR1 = sq root of ((RL*RL) - (RD*RD))
• PR2 = PR1 - RR
• DST = ST - ((PR2 + 1/2ST) - RL)

This result is the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length, above, for calculating the DCR.


The practical range of dynamic compression ratios for a street engine is 8.0:1 - 8.75:1, with 8.4:1 being the limit for non-professional builds. Careful machining and good design with careful attention to quench dimensions allow up to 8.75:1 to run on 93 PON fuel. The above assumes that the engine will be running at incipient detonation when under extreme loads, and also assumes that the spark advance program has been optimized to result in maximum cylinder pressure to occur at or near 15 degrees ATDC as often as possible throughout its rev and load range.

Although much more meaningful than SCR analyses, the above applies only to naturally aspirated engines and begins to lose accuracy as engine speeds increase, especially those equipped with high overlap camshafts, which employ wave dynamics to "supercharge" the combustion chambers at higher RPMs.